Various forms of cruise control systems are known. For instance, a copending U.S. patent application Ser. No. 129,257, filed Dec. 7, 1987, discloses such a cruise control system having improved self diagnosis features. In a typical cruise control system, once the set switch is pressed after the vehicle speed has reached a desired level as selected by the driver by appropriately depressing an accelerator pedal, an actuator under the control of a control system takes over the control of the accelerator pedal and adjusts the depression of the accelerator pedal so as to maintain the set speed of the vehicle from then on without requiring any effort from the driver.
Such an actuator may consist of a vacuum actuator deriving its power from the vacuum in the intake manifold of an internal combustion engine or, alternatively, of an electric motor provided with an electromagnetic clutch. An actuator consisting of an electric motor has an advantage of compactness and accuracy of control. However, supply of electric power to the motor should be discontinued when the throttle valve is fully closed, not only for avoiding the waste of electric power but also for preventing the overloading of the electric motor which might impair the durability of the electric motor. This can be easily accomplished by providing a limit switch which detects the state of the actuator corresponding to the fully closed state of the throttle valve and terminating the supply of electric power to the motor when the limit of the decelerating action of the actuator has been detected by this limit switch.
Since the soundness of this limit switch is important in view of the fact that the electric motor would be placed in an unexpected overload condition, should the limit switch fail to detect the fully closed state of the throttle valve, a certain measure must be taken to prevent any ill effects which might arise as a result of a faulty action of the limit switch. Furthermore, since the limit of the deceleration stroke of the actuator serves as a reference point in controlling the action of the actuator, any failure in this limit switch could cause an undesirable faulty behavior of the actuator.